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Tuesday, December 06, 2016

Protecting and reactivating memories

I want to point to two recent Science Magazine articles on memory. From the magazine's summaries..

Protecting memories from stress

It is widely accepted that stress has a negative impact on memory retrieval. But specific approaches to learning can counteract this effect. Smith et al. found that when memory was tested immediately after the onset of stress, stress effects were reduced. Furthermore, when subjects learned novel material by using a highly effective learning technique involving practice tests, their memory was also protected against the negative effects of stress.

More than a decade of research has supported a robust consensus: Acute stress impairs memory retrieval. We aimed to determine whether a highly effective learning technique could strengthen memory against the negative effects of stress. To bolster memory, we used retrieval practice, or the act of taking practice tests. Participants first learned stimuli by either restudying or engaging in retrieval practice. Twenty-four hours later, we induced stress in half of the participants and assessed subsequent memory performance. Participants who learned by restudying demonstrated the typical stress-related memory impairment, whereas those who learned by retrieval practice were immune to the deleterious effects of stress. These results suggest that the effects of stress on memory retrieval may be contingent on the strength of the memory representations themselves.

How to reactivate forgotten memories

Sophisticated techniques can decode stimulus representations for items held in a person's working memory. However, when subjects shift their attention toward something else, the neural representation of the now unattended item drops to baseline, as though the item has been forgotten. Rose et al. used single-pulse transcranial magnetic stimulation (TMS) to briefly reactivate the representation of an unattended item. A short pulse of TMS enhanced recognition of “forgotten” stimuli, bringing an unattended item back into focal attention.

The ability to hold information in working memory is fundamental for cognition. Contrary to the long-standing view that working memory depends on sustained, elevated activity, we present evidence suggesting that humans can hold information in working memory via “activity-silent” synaptic mechanisms. Using multivariate pattern analyses to decode brain activity patterns, we found that the active representation of an item in working memory drops to baseline when attention shifts away. A targeted pulse of transcranial magnetic stimulation produced a brief reemergence of the item in concurrently measured brain activity. This reactivation effect occurred and influenced memory performance only when the item was potentially relevant later in the trial, which suggests that the representation is dynamic and modifiable via cognitive control. The results support a synaptic theory of working memory.